1. Field of the Invention
This invention relates to close-range radio communications in general. More specifically, this invention relates to a radio system for data transmission by microwaves between a central station and a passive end that does not have its own hyperfrequence source and that operates in a response mode.
Such a radio system can be used to exchange information over a short distance between a main point, called central station, and one or more passive ends, called terminal stations, which should be very simply designed to meet cost and space requirements. This type of radio system has numerous applications, e.g. in:
remote-controlled data gathering systems such as a remote-controlled domestic counter reading system with a mobile radio accessor, PA1 access or checking management systems with remote inquiry for code numbers memorized in individual access or checking cards, PA1 road or rail signalling systems, PA1 automatic sorting systems, e.g. for parcel post, PA1 automatic invoicing systems for contact-free chip cards. PA1 a two-dimensional emission antenna with two square two-dimensional elements and having first two ports receiving first and second microwave signals corresponding to the first and second microwaves for emitting the first microwave to the terminal station during the first data transmission periods and for emitting the second microwave and for supplying the second microwave signal during the second data transmission periods, a two-dimensional reception antenna with two square two-dimensional elements for receiving the third microwave emitted by the terminal station the during second data transmission periods thereby delivering two phase-shifted signals corresponding to the third microwave, and means connected to the square elements of the transmission and reception antennae and receiving the two phase-shifted microwave signals and the second microwave signal for phase demodulating the second data signal during the second data transmission periods.
2. Description of the Prior Art
There are several known radio systems with a passive end in the prior art. French patent No. 2,527,870 describes a coded response card in which usually, an amplitude modulation is used to transmit data in the transmission direction going from the passive terminal station to the central station. An incident microwave transmitted by the central station is received by the passive terminal station in which data to be transmitted modulate in amplitude the incident microwave received or one of its harmonics before emitting it to the central station. The passive terminal station is supplied by the incident microwave. A rectifying and filtering circuit is usually provided to produce a d.c. supply voltage from the received incident microwave.
The modulated microwave retransmitted by the terminal station to the central station has a low output due to the low energy output rate at reception of the incident microwave and to the energy deducted to supply the passive end, and consequently the range of such a radio system is necessarily limited. Moreover, the amplitude modulation has low immunity from noise compared to a frequency or phase modulation, which does not favour the range of such a system.
A solution to improve the range of a radio system with a passive terminal station will be e.g. to replace the amplitude modulation of the retransmitted microwave by a two-phase-state modulation providing better immunity from noise.
Our U.S. patent application Ser. No. 351,234 filed May 15, 1989, now U.S. Pat. No. 4,956,645, claiming the French priority N.degree.88-06794 filed May 20, 1988 and published Nov. 22, 1989 under N.degree. 2,631,725, describes a radio system in which the microwave retransmitted by the passive terminal station is modulated in two phase states. This radio system enables communications of good quality. However, its cost is relatively high, mainly due to the necessity of a microwave switch in the passive terminal station. It is desirable to provide a radio system with a less expensive passive terminal station, operating in phase modulation for the retransmitted microwave, though paying the price of lesser performances relative to the radio system embodying the above-mentioned patent application.